Electrodeposited metallic coatings applied by selective and/or brush plating are extensively used in consumer and industrial applications. In brush plating, dimensionally stable anodes (DSA) made of graphitic materials are commonly used. However, in the case of electrolytes that contain ions that can be oxidized (such as chlorides, phosphorus-bearing ions, or metal ions with multiple valence states), significant challenges are encountered leading to (i) undesired chlorine gas evolution posing health and safety risks, (ii) a rapid deterioration of the electrolyte, and (iii) the inability to maintain a constant coating composition with increasing deposition time. These problems may be caused by anodic reactions, including but not limited to the oxidation of hypophosphorous or phosphorous ions to phosphoric ions, chloride to chlorine, Fe2+ to Fe3+, and water to oxygen gas.
It is well documented that DSAs and consumable anodes (SAs) are used in electrodeposition. Where feasible, e.g., in tank, drum and barrel plating, consumable anodes containing the metal or an alloy of the elements cathodically deposited are frequently used. In this case metal chips, rounds or pieces are usually filled into suitable anode cages made of inert materials such as titanium baskets. In contrast DSAs are used in commercial brush-plating applications.
Prior art specific to selective plating includes the disclosure of brush or tampon plating tools employing “anode brushes” which are wrapped in an absorbent tool cover material or felt. The brush is rubbed over the surface to be plated and electrolyte solution is injected into tool such that it must contact the anode and pass through the absorbent tool cover material. Typical anodes are made of graphite and serve as dimensionally stable anodes (DSAs), i.e., apart from corrosion or undesired mechanical degradation, these anodes are not consumed during the plating process and do not liberate metal ions used for the cathodic deposition.
In brush electroplating consumable anodes, which contain the very metal/alloy to be plated and replenish the cathodically reduced/deposited metal ions via anodic dissolution, are not used. Reasons include added complexity due to size/shape changes associated with consumable anodes and the confined geometry of the “electrolytic cell”.
Icxi in U.S. Pat. No. 2,961,395 discloses a process for electroplating an article without the necessity to immerse the surface being treated into a plating tank. The hand-manipulated applicator serves as an anode and applies chemical solutions to the metal surface of the workpiece to be plated. The active anode is made of carbon. The workpiece to be plated serves as a cathode. The hand applicator anode with the wick containing the electrolyte and the workpiece cathode are connected to a DC power source to generate a metal coating on the workpiece by passing a DC current.
Smith in U.S. Pat. No. 4,931,150 discloses a selective electroplating apparatus for rapidly depositing a metal onto a selected surface of a workpiece employing conformal consumable or non-consumable anodes.
Moskowitz in U.S. Pat. No. 5,409,593 discloses a device for brush electroplating a surface of a workpiece using a consumable anode. The anode is selectively retained within a cavity formed in a lower surface of a carrier piece composed of a generally electrically non-conductive material. The lower surface of the carrier piece is shaped to conform to at least a portion of the surface of the workpiece. An absorbent material extends over the lower surface of the carrier piece to form a brush. The cover material and lower surface of the anode are spaced from each other to form an electrolyte chamber. The device also includes an assembly that is fluidly connected to the inter-electrode gap to inject a flow of the electrolyte into the chamber. The metal anode plate insert can be mechanically readjusted/lowered in the anode tool (to account for increasing anode depletion).
Many commercial electrolytes contain chloride ions (e.g., Watts bath for Ni and/or Co). On graphite or other active anode materials that are typically employed in brush plating, chlorine is anodically evolved in addition to or instead of oxygen. A number of industrially popular metallic coatings include phosphorus as an alloying element which poses significant bath management challenges and coating composition uniformity issues when using DSAs. Other electrolytes contain metal-ions that can be anodically oxidized when employing non-consumable anodes resulting in difficulties, e.g., the Fe2+/Fe3+ reaction in Fe containing electrolytes. The prior art is rich in the use of P-bearing electrodeposited coatings comprising Ni-, Co-, and/or Fe-based alloy coatings.
Brenner in U.S. Pat. No. 2,643,221 discloses the electrodeposition of Ni—P (with up to 15% P) and Co—P (up to 10% P) alloy coatings from solutions containing the metal ions, chlorides, and phosphoric and phosphorous acid. Brenner is silent on the use of selective and brush plating.
Engelhaupt in U.S. Pat. No. 6,406,611 describes electrodeposited Ni or Co alloys with 2at.% to 25at.% P alloys having low-stress from sulfate electrolytes containing phosphorous acid and using consumable or insoluble anodes. Engelhaupt is silent on the use of selective and brush plating.
Ware in US 2005/0170201 and US 2007/0084731 describes coarse-grained Co—P—B coatings of low compressive residual stress and improved fatigue resistance using soluble or insoluble noble metal anodes and an electrolyte containing, among other, chloride, sulfate and phosphorous ions. Ware is silent on the use of selective and brush plating.
Palumbo in US 2005/0205425 and DE 10228323, assigned to the same assignee as the present application, discloses a process for forming coatings or freestanding deposits of nanocrystalline metals, metal alloys or metal matrix composites. The process employs tank, drum plating or selective plating processes including brush plating using aqueous electrolytes and optionally a non-stationary anode or cathode. Nanocrystalline metal matrix composites are disclosed as well. Palumbo teaches that the electrolyte flow rate normalized for electrode area can be used to control the microstructure of the cathodic deposit. Specifically, grain refinement is achieved above critical normalized agitation rates.
Palumbo in US 2003/0234181, assigned to the same assignee as the present application, discloses a process for electroforming in situ a structural reinforcing layer of selected metallic material for repairing an external surface area of a degraded section of metallic workpieces. A suitable apparatus is assembled on or near the degraded site and is sealed in place to form the plating cell. Also described is a process for plating “patches” onto degraded areas by selective plating including brush plating.
Facchini in US 2010/0304172, US 2010/0304179 and US 2010/0304182 describes the electrodeposition of coatings or free-standing components comprised of Co-bearing metallic materials, including Co—P, that possess a fine-grained and/or amorphous microstructure with improved fatigue performance using soluble or dimensionally stable anodes and tank, drum, barrel and brush plating.
Hamano in U.S. Pat. No. 4,765,872 describes a method for treating a plating solution containg Fe3+ ions in a separate electrolytic cell having a cathode compartment and an anode compartment partitioned by an ion-exchange membrane. Plating solution containing up to 10 g/l of Fe3+ ions is pumped into the cathode compartment, an electrically conductive solution is provided to the anode compartment, and Fe3+ ions are electrolytically reduced in the plating solution to Fe2+ ions using a cathode having a hydrogen overvoltage of not higher than 350 mV, preferably made of a carbon material.